Medical ultrasound imaging systems typically use a one-dimensional phased array to form an image of a two-dimensional slice through a patient's body. This approach has limitations. First, the two-dimensional slice is always perpendicular to the face of the transducer, thereby limiting the choice of views. For example, a cardiologist sometimes wants to view heart valves in plane. This requires a double oblique imaging plane with respect to the face of the transducer. This plane can only be derived from three-dimensional data. Second, anatomy such as the left ventricle is inherently three-dimensional. To obtain an accurate volume measurement of the left ventricle, three-dimensional data must be acquired.
Current methods used to acquire three-dimensional data, such as may be obtained using Hewlett-Packard's Omni Plane transducers, use a one-dimensional array that is mechanically moved in a second dimension. This method may require several minutes to obtain a three-dimensional data set. Furthermore, the organs of interest may move during acquisition of the three-dimensional data set.
Phased array ultrasound transducers having multiple elements in the azimuth direction and a few elements in the elevation direction permit scanning in the azimuth direction and elevation focusing. See for example, U.S. Pat. No. 5,462,057 issued Oct. 31, 1995 to Hunt et al. These transducer configurations, often referred to as 1.5 dimensional arrays, do not permit beam steering in the elevation direction.
A system capable of acquiring real time three-dimensional data by electronically steering in two dimensions is described by T. Ota in "Accuracy of Left Ventricular Stroke Volume Measurement Using Real-Time, Three Dimensional Echocardiography Flow Probe in Vivo", 70th Scientific Session American Heart Association Meeting, Nov. 11, 1997. This system uses 512 active transducer elements. Signals from the transducer elements are passed through a cable having 512 coaxial conductors into a system with appropriate electronics. The image quality of the system is limited due to the small number of transducer elements used. Furthermore, since the cable between the transducer and the system has a significant diameter, it is unlikely that this technology can be extended to many more transducer elements without an unacceptably large cable or a cable with such small diameter conductors that significant signal loss will occur.
A two-dimensional phased array ultrasound imaging system wherein signal delays are distributed between a probe and a base station are described in U.S. Pat. No. 5,229,933 issued Jul. 20, 1993 to Larson, III et al.
A portable ultrasound imaging system wherein a handheld scan head enclosure houses an array of ultrasonic transducers, transmit circuitry and beamforming circuitry is disclosed in U.S. Pat. No. 5,590,658 issued Jan. 7, 1997 to Chiang et al. It is not considered feasible to incorporate all transmitting circuitry and beamforming circuitry for a three-dimensional phased array scanner into a handheld scan head of practical size.
An ultrasound beamformer which utilizes subarray processors to reduce the cost, power and size of digital beamformers is disclosed in U.S. Pat. No. 5,573,001 issued Nov. 12, 1996 to Petrofsky et al. Each subarray processor includes at least one phase shifter and a summer. Each phase shifter is responsive to at least one of the transducer signals to shift the transducer signal by a respective phase angle and to apply the phase shifted transducer signals to the summer. The summed subarray signals are applied to a beamformer processor. The disclosed beamformer is used for two-dimensional imaging.
A device for a three-dimensional focusing of an ultrasonic beam is disclosed in U.S. Pat. No. 5,027,820 issued Jul. 1, 1991 to Pesque. The device includes a cylindrical phased array.
None of the known prior art ultrasound imaging techniques have achieved high quality three-dimensional ultrasound imaging with transducer assemblies that are practical in size, cost and complexity.